1. Field of Invention
The present invention relates to transportable systems having both direct and projection viewing modes of operation, and to electro-optical backlighting panels for use therein.
2. Brief Description of the State of the Art
Presently, most portable computing systems include a flat liquid crystal display (LCD) panel for directly viewing video imagery displayed thereon. Portable computer systems of this type include notebook, laptop, and palmtop computers.
In general, prior art LCD display panels have essentially the same basic construction in that each includes a conventional backlighting structure affixed to the rear surface of either a passive or active matrix LCD panel. Several different backlighting panel designs are described in the technical paper "New Backlighting Technologies for LCDs" by Kevin J. Hathaway, et al., published at pages 751-754 in SID 91 Digest. In recent times, the "light pipe" backlight design, in particular, has been widely used in many commercially available notebook computers.
Specifically, prior art "light pipe" backlight assemblies are constructed from a rectangularly shaped light guiding panel, typically fabricated from an acrylic plastic sheet having a thickness of about 4 millimeters or so. Along the opposite side edges of the acrylic sheet, a pair of miniature fluorescent light tubes are mounted within suitably designed light reflective mounts. The function of the fluorescent light tubes is to produce and direct incoherent light into the interior of the light guiding panel within which the light is typically bounded by the well known principle of "total internal reflection". Under ideal conditions, light will not leak out of the surfaces of the acrylic plastic sheet. However, light can be extracted or leaked out from these surfaces by forming therein scratches, undulations, or any other means of locally altering the critical angle for total internal reflection. By achieving light extraction in this manner, the backlighting panel can be used to illuminate an LCD panel.
In order to compensate for the decrease in light intensity in the light guiding panel at distances away from the fluorescent tubes, a light extracting pattern is permanently formed on one or both surfaces of the light guiding panel. Typically the light extracting pattern is realized as a dot pattern permanently embossed or sand-blasted upon the front surface of the acrylic light guiding panel. The density of the dot pattern is made to increase quadratically with distance from the fluorescent light tubes in order to achieve light intensity compensation along the light guiding panel. With this construction, a constant backlighting brightness is maintained across the light guiding panel.
In order to integrate (i.e. diffuse) the spotted distribution of light emanating from the light extracting pattern towards the LCD panel, a first light diffusing structure is placed on top of the light guiding panel. Typically, the first light diffusing structure is made from one or more thin sheets of translucent plastic attached to the front surface of the light guiding panel. In most commercial "light pipe" backlight designs, a second light diffusing structure is placed over the rear surface of the light guiding panel to diffuse the spotted distribution of light emanating from the permanently formed light extracting pattern towards the reflective surface disposed behind the light guiding panel. Typically, the second light diffusing structure is made from one or more thin sheets of translucent plastic attached to the rear surface of the light guiding panel. Together, the light guiding panel, fluorescent light tubes, light diffusing sheets and the light reflective layer cooperate to produce a plane of backlight having a uniform spatial intensity for optical processing by the LCD panel affixed to the backlighting panel.
While the prior art backlighting panel design described above has proven useful in the direct viewing of visual imagery on LCD display screens, its permanently formed light extracting pattern renders it unsuitable in projection viewing modes of operation. This fact is best illustrated by example.
In the recently introduced notebook computer, marketed under the tradename "Cruiser.TM." by EMCO/REVERED Technologies, Inc. and generally described in U.S. Pat. No. 5,353,075 to Conner, et al., the above-described "light pipe" backlighting panel design is used to construct a portable computer system having both direct and projection viewing modes of operation. In the direct viewing mode, the prior art backlighting panel is positioned against the active-matrix LCD panel. Each time the user desires to operate the notebook computer in its projection viewing mode, the user must mechanically reconfigure the Cruiser.TM. notebook computer by physically removing the prior art backlighting panel in order to reveal the active matrix LCD panel, and provide an optically clear path for the light rays to pass therethrough.
Recently, Intellimedia, Corporation of Benton Harbor, Michigan has introduced the IntelliMedia.TM. Multimedia Presentation System which consists of portable computer system having a flat LCD projection panel which can support both direct and projection viewing modes. In the direct viewing mode, the user of this system is required to affix an auxiliary backlighting panel beneath the LCD panel. Then, when operated in the projection viewing mode, the user is required to remove the auxiliary backlighting panel, and place the LCD panel upon an external overhead projector, much like that required by the Cruiser.TM. computer system.
While the above-described image display systems provide both direct and projection viewing modes, they both nevertheless suffer from a number of serious shortcomings and drawbacks which make them less than commercially attractive products.
In particular, the need to physically remove the entire backlighting panel from the Cruiser.TM. computer during its projection viewing mode, poses a substantial risk of damage to the backlighting panel and imposes an added responsibility upon the user to safely store the same when the computer system is operated in its projection viewing mode. Also, from a practical standpoint, the need to place the display panel assembly of the Cruiser.TM. computer and the entire display panel of the Intellimedia.TM. system upon an overhead projector during projection viewing, necessitates that the user either tote an overhead projector along with such prior art systems, or have access to one during projection viewing. Consequently, such prior art image display systems lack the versatility of operation in either direct or projection modes of viewing, and thus are incapable of functioning as truly portable systems.
Thus, there is a great need in the art for an improved image display system which has direct and projection viewing modes, without the shortcomings and drawbacks of the prior art systems.